feat: allow bv_decide users to configure the SAT solver (#11847)

This PR adds a new `solverMode` field to `bv_decide`'s configuration,
allowing users to configure
the SAT solver for different kinds of workloads.

src/Lean/Elab/Tactic/BVDecide/External.lean

@@ -8,6 +8,7 @@ module
 prelude
 public import Std.Tactic.BVDecide.LRAT.Parser
 public import Lean.CoreM
+public import Std.Tactic.BVDecide.Syntax
 
 public section
 
@@ -146,7 +147,7 @@ solvers the solver is run with `timeout` in seconds as a maximum time limit to s
 Note: This function currently assume that the solver has the same CLI as CaDiCal.
 -/
 def satQuery (solverPath : System.FilePath) (problemPath : System.FilePath) (proofOutput : System.FilePath)
-    (timeout : Nat) (binaryProofs : Bool) :
+    (timeout : Nat) (binaryProofs : Bool) (mode : Frontend.SolverMode) :
     CoreM SolverResult := do
   let cmd := solverPath.toString
   let mut args := #[
@@ -156,17 +157,12 @@ def satQuery (solverPath : System.FilePath) (problemPath : System.FilePath) (pro
     s!"--binary={binaryProofs}",
     "--quiet",
     /-
-    This sets the magic parameters of cadical to optimize for UNSAT search.
-    Given the fact that we are mostly interested in proving things and expect user goals to be
-    provable this is a fine value to set
-    -/
-    "--unsat",
-    /-
     Bitwuzla sets this option and it does improve performance practically:
     https://github.com/bitwuzla/bitwuzla/blob/0e81e616af4d4421729884f01928b194c3536c76/src/sat/cadical.cpp#L34
     -/
     "--shrink=0"
   ]
+  args := args ++ solverModeFlags mode
 
   -- We implement timeouting ourselves because cadicals -t option is not available on Windows.
   let out? ← runInterruptible timeout { cmd, args, stdin := .piped, stdout := .piped, stderr := .null }
@@ -190,6 +186,12 @@ def satQuery (solverPath : System.FilePath) (problemPath : System.FilePath) (pro
           throwError s!"Error {err} while parsing:\n{stdout}"
       else
         throwError s!"The external prover produced unexpected output, stdout:\n{stdout}stderr:\n{stderr}"
+where
+  solverModeFlags (mode : Frontend.SolverMode) : Array String :=
+    match mode with
+    | .proof => #["--unsat"]
+    | .counterexample => #["--sat"]
+    | .default => #["--default"]
 
 end External
 

src/Lean/Elab/Tactic/BVDecide/Frontend/BVDecide.lean

@@ -344,7 +344,14 @@ def lratBitblaster (goal : MVarId) (ctx : TacticContext) (reflectionResult : Ref
 
   let res ←
     withTraceNode `Meta.Tactic.sat (fun _ => return "Obtaining external proof certificate") do
-      runExternal cnf ctx.solver ctx.lratPath ctx.config.trimProofs ctx.config.timeout ctx.config.binaryProofs
+      runExternal
+        cnf
+        ctx.solver
+        ctx.lratPath
+        ctx.config.trimProofs
+        ctx.config.timeout
+        ctx.config.binaryProofs
+        ctx.config.solverMode
 
   match res with
   | .ok cert =>

src/Lean/Elab/Tactic/BVDecide/Frontend/LRAT.lean

@@ -131,7 +131,7 @@ Run an external SAT solver on the `CNF` to obtain an LRAT proof.
 This will obtain an `LratCert` if the formula is UNSAT and throw errors otherwise.
 -/
 def runExternal (cnf : CNF Nat) (solver : System.FilePath) (lratPath : System.FilePath)
-    (trimProofs : Bool) (timeout : Nat) (binaryProofs : Bool) :
+    (trimProofs : Bool) (timeout : Nat) (binaryProofs : Bool) (solverMode : Frontend.SolverMode) :
     CoreM (Except (Array (Bool × Nat)) LratCert) := do
   IO.FS.withTempFile fun cnfHandle cnfPath => do
     withTraceNode `Meta.Tactic.sat (fun _ => return "Serializing SAT problem to DIMACS file") do
@@ -141,7 +141,7 @@ def runExternal (cnf : CNF Nat) (solver : System.FilePath) (lratPath : System.Fi
 
     let res ←
       withTraceNode `Meta.Tactic.sat (fun _ => return "Running SAT solver") do
-        External.satQuery solver cnfPath lratPath timeout binaryProofs
+        External.satQuery solver cnfPath lratPath timeout binaryProofs solverMode
     if let .sat assignment := res then
       return .error assignment
 

src/Std/Tactic/BVDecide/Syntax.lean

@@ -14,6 +14,23 @@ set_option linter.missingDocs true -- keep it documented
 
 namespace Lean.Elab.Tactic.BVDecide.Frontend
 
+/--
+The various kinds of configurations offered for the SAT solver.
+-/
+inductive SolverMode where
+  /--
+  Set SAT solver options to improve proof search.
+  -/
+  | proof
+  /--
+  Set SAT solver options to improve counterexample search.
+  -/
+  | counterexample
+  /--
+  Don't set additional SAT solver flags.
+  -/
+  | default
+
 /--
 The configuration options for `bv_decide`.
 -/
@@ -70,6 +87,11 @@ structure BVDecideConfig where
   if matching multiplications are not needed to proof a goal.
   -/
   shortCircuit : Bool := false
+  /--
+  The SAT solver configuration to use. Defaults to `.proof` as that is the most relevant use case
+  for `bv_decide`.
+  -/
+  solverMode : SolverMode := .proof
 
 end Lean.Elab.Tactic.BVDecide.Frontend
 

tests/lean/run/bv_decide_solver_modes.lean

@@ -0,0 +1,38 @@
+import Std.Tactic.BVDecide
+
+
+/--
+error: The prover found a counterexample, consider the following assignment:
+x = 4294967295#32
+y = 4294967295#32
+-/
+#guard_msgs in
+example (x y : BitVec 32) : x * y = y + x := by
+  bv_decide
+
+/--
+error: The prover found a counterexample, consider the following assignment:
+x = 4294967295#32
+y = 4294967295#32
+-/
+#guard_msgs in
+example (x y : BitVec 32) : x * y = y + x := by
+  bv_decide (config := { solverMode := .counterexample })
+
+/--
+error: The prover found a counterexample, consider the following assignment:
+x = 4294967295#32
+y = 4294967295#32
+-/
+#guard_msgs in
+example (x y : BitVec 32) : x * y = y + x := by
+  bv_decide (config := { solverMode := .default })
+
+/--
+error: The prover found a counterexample, consider the following assignment:
+x = 4294967295#32
+y = 4294967295#32
+-/
+#guard_msgs in
+example (x y : BitVec 32) : x * y = y + x := by
+  bv_decide (config := { solverMode := .proof })